1. Field of the Invention
The present invention generally relates to a digital data recording/reproducing apparatus for recording digital data at a high recording density, and particularly to a data discrimination apparatus for correcting a reproduced signal influenced by the interference occurring between adjacent bits, an amount of which varies depending upon input data patterns (a frequency of changes of the signal level or an interval between a symbol "1" and a subsequent symbol "1").
2. Description of the Related Art
In a digital data recording/reproducing apparatus, such as a magnetic recording apparatus using a disk-like recording medium (i.e., inter-symbol), it is known that a reproduced signal waveform suffers from a non-linear distortion or decrease of amplitude thereof because of a so-called inter-symbol interference occurring between adjacent bits which are in close proximity to each other in the recorded signal. This may be particularly significant when the recording density of the medium becomes higher.
A waveform equalizing technique used in an adaptive equalizer or a decision-feedback equalizer is a prior art approach to compensate for the non-linear distortion (e.g., a horizontally non-symmetrical waveform) of the signal waveform and the decrease in amplitude due to the interference. An example of the adaptive equalizer is shown in Japanese patent application laid-open (KOKAI) No. 4-207708, in which when a code in an output signal from a transversal filter is different from an immediately preceding or succeeding code, a decision error is derived from the output signal to update tap coefficients of the equalizer. An example of the decision-feedback equalizer is shown in Japanese patent application laid-open (KOKAI) No. 3-284014, in which each of the tap coefficients is determined and corrected by using the LMS (Least Mean Square) algorithm based on an error signal between the input and the output of a decision unit, and signals of respective taps of forward and backward equalizers.
Referring to FIG. 14, there is shown a construction of such an adaptive equalizer. Representing an input, an output and tap coefficients of the equalizer by "x", "y" and "h", respectively, an assumption is made such that input and output data x and y are considered as data at the same time point, with "k" being a reference time point. The block labeled "ADAPTIVE ALGORITHM" serves to update the tap coefficients h.sub.0 -h.sub.N-1 based on error data e(k)=d(k)-y(k), where d(k) indicates an expected value. It is also assumed that no clock delay occurs in this block. An output of the equalizer obtained based on the thus updated tap coefficients h.sub.0 -h.sub.N-1 is data y(k+1) at a time of one clock later, which corresponds to input data x(k+1).
These prior art techniques have the following drawbacks: With the conventional recording density of about 50k fci (flux change per inch), whether data has loose or fine intervals of adjacent bits makes almost no difference to an amount of interference. However, when the recording density becomes higher and higher in future, the interference occurring at the fine interval becomes greater, whereas that at the loose interval remains intact. Therefore, an amount of interference varies depending upon the input data patterns, causing larger variations of the non-linear distortion of the signal waveform and a decrease of the amplitude.
As mentioned above, the prior art equalizer controls the tap coefficients (i.e., equalizer characteristics) so as to minimize an error between the expected value and the equalizer output. Feedback is effected not from the data used for the decision, i.e., the expected value and the equalizer output, but from data at a time after the reference time, x(k+1). Here, the term "feedback" is used to represent that the tap coefficients are updated to be used to affect the equalizer output. With this arrangement of the prior art equalizer, it is impossible to correct, in a bit-by-bit manner, the non-linear distortion or decrease in amplitude of the signal waveform which varies depending upon the input data patterns. In this specification, "adjacent bits" refer to two or more symbol "1"'s which are close with each other within a range of a certain number of bits.